In recent years, there has been considerable interest in the production of glass fibers. Due to the tremendous usages of glass fibers, this interest has been particularly focused on increasing the production of individual fiber forming stations.
In the production of fibers, molten glass is typically passed through tips or orifices in a bushing to create individual fibers. As the molten streams of glass flow through the tips or orifices for attenuation into fibers, the fiber forming environment below the bushing must be carefully controlled for a stable fiber forming operation.
Glass fiber forming bushings today conventionally have a plurality of tips projecting below the bushing floor through which streams of molten glass flow. Cones of glass form at the exit area of each tip and fibers are attenuated therefrom. The fiber forming environment in the cone region must be carefully controlled. Conventionally, this is done by placing solid metallic heat exchanging units or finshields beneath the bushing and between rows of tips. Such finshield units have been used for many years to control the fiber forming region beneath a bushing. Such a heat exchanging device is described in U.S. Pat. No. 2,908,036.
Over the years, the number of fibers produced by a single bushing has increased greatly. In the past it was common for a bushing to produce about 200 fibers. Today bushings can produce 2000, or more, fibers. As the number of fibers per bushing continues to increase, problems with conventional fiber forming processes and apparatus have arisen. When using conventional finshield units to control the fiber forming environment, one is limited in the tip density that can be employed in the fiber forming bushing as there must be sufficient space for the fin members to project between rows of tips. Thus, to increase the number of fibers being formed by a single bushing, the physical size of the bushing must be increased. With bushings of increased size and increased glass throughput per tip, solid finshield members as described in U.S. Pat. No. 2,908,036 are operated at the very upper limit of their physical heat transfer capability.
As bushings are produced to create even larger numbers of fibers per bushing and as the throughput per bushing tip or orifice is increased, environmental control by conventional finshield units can be inadequate. There has been considerable activity in the glass fiber forming field to develop a process and apparatus for controlling the fiber forming environment in such bushings.
One such glass fiber forming process is described in U.S. Pat. No. 3,905,790. In this patent, the need for conventional finshields is eliminated. This patent utilizes a bushing having a flat orifice plate with closely packed non-tipped orifices and a lower air nozzle from which an upwardly directed flow of air issues to impinge directly on the orifice plate. The patent teaches that the orifice density of such a bushing can be greatly increased over that of a conventional bushing using conventional finshields. The impingement of the cooling air directly upon the orifice plate to flow outwardly along the plate cools the molten glass cones to maintain fiber separation and to eliminate any stagnant air at the under surface of the plate. Problems can arise in maintaining a stable glass fiber forming operation with this process. Problems can also arise in restarting this process after there is an interruption in the formation of fibers. After a fiber forming interruption a tipless bushing does not form beads of glass at each orifice as does a tipped bushing. Restarting such an orificed bushing requires a very skilled operator and the restarting operation is quite time-consuming.
Improvements in the glass fiber forming process and apparatus are desired.